First Quantum Theory : Black Body Radiation by Max Planck

Quantum Mechanics is the branch of the physics relating to very small. Quantum mechanics explains the behaviour of matter and its interactions with energy on the scale of atoms and subatomic particles.

How the Quantum Physics born? Well, it’s very hard to answer. In the late 19th century, thermal radiation had been fairly well characterized experimentally. The black-body radiation by Max Planck is the first Quantum theory. Max Planck describe that the electromagnetic radiation emitted by a black body.

“A black body emits radiation called black body radiation.” Planck described the radiation by assuming that radiation was emitted in quanta. Quanta is the minimum amount of any physical entity involved in an interaction.

Max Planck, in 1901, accurately described the radiation by assuming that electromagnetic radiation was emitted in quanta.

According to Plank any black body in thermal equilibrium emits the electromagnetic radiation. This radiation is called black body radiation. This emitted radiation has a specific spectrum and intensity which is depends only on the temperature of the black body.

So the basically the black body is an idealized physical body. It means the body absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Here note that the black body is a theoretical only concept, there’s nothing like it exists, but you can find approximate realizations of black body in real world.

Explaining the properties of black-body radiation was a major challenge in theoretical physics during the late nineteenth century. Predictions was based on classical theories which are failed to explain black body spectra observed experimentally, especially at shorter wavelength. But in 1901 it was solved.

Planck’s law explains increasing temperature of a body allows it to emit more energy overall, and means that a larger proportion of the energy is towards the violet end of the spectrum.

The first model which was able to explain the full spectrum of thermal radiation was created Max Planck. He gave a mathematical formula in which the thermal radiation was in equilibrium with a set of harmonic oscillators.

However to reproduce the results, Max assumed that each of oscillator emitted an integer number of units of energy at its single characteristic frequency, rather than being able to emit any arbitrary amount of energy. In simple words, the energy emitted by an oscillator was quantized.

The quantum of energy for each oscillator, according to Planck, was proportional to the frequency of the oscillator; the constant of proportionality is now known as the Planck constant. The Planck constant, usually written as h, has the value of 6.626 × 10-34 joule second. The energy E of an oscillator of frequency f is given by

E=nhf , where n=1,2,3,…

To change the colour of such a radiating body, it is necessary to change its temperature

Note that the spectral radiance depends on two variables, wavelength and temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body. Despite its simplicity, Planck’s law describes radiation properties of objects (e.g. our body, planets, stars) reasonably well.

Planck’s quantum hypothesis is a pioneering work, heralding advent of a new era of modern physics and quantum theory.